LTC3524_15 Datasheet, PDF (7/16 Pages) Linear Technology – Adjustable TFT Bias Supply with WLED Driver

LTC3524
LCD BIAS PIN FUNCTIONS
FBN (Pin 13): Feedback Pin for the VN Charge-Pump
Output. Reference voltage is 1.0V. Connect the resistive
divider tap between V_OUT and VN here with minimum
trace area.
VN = ^{−R6 (VOUT − 1)} + 1 (See Block Diagram)
R5
FBH (Pin 14): Feedback Pin for the VH Charge-Pump
Output. Reference voltage is 1.225V. Connect resistive
divider tap here with minimum trace area.
VH
=
1.225
⎛
⎝⎜
1+
R3 ⎞
R4 ⎠⎟
(See Block Diagram)
VH (Pin 15): Charge Pump Quadrupler Output. This output
can be regulated to 4X V_OUT and is capable of delivering
up to 2mA to a load. VH should be bypassed to GND with
a 0.47μF X5R type ceramic capacitor. Connect V2x to
V_OUT for applications requiring a regulated voltage less
than 2X V_OUT.
CH⁺ (Pin 16): Charge Pump Quadrupler Flying Capacitor
Positive Node. The charge pump quadrupler (4X) flying
capacitor is connected between CH⁺ and CH^–. The voltage
on CH⁺ will alternate between V2x and VH at an approximate
50% duty cycle while the charge pump is operating. Use
a 0.1μF X5R type ceramic capacitor for best results.
CH^– (Pin 17): Charge Pump Quadrupler (4X) Flying Ca-
pacitor Negative Node. The voltage on CH^– will alternate
between GND and V2x at an approximate 50% duty cycle
while the charge pump is operating. Use a 0.1μF X5R type
ceramic capacitor for best results.
WHITE LED DRIVER PIN FUNCTIONS
LED2 (Pin 18): Output for Second LED String. Connect
up to five white LEDs between LED2 (anode) and GND
(cathode). For best current matching and efficiency use
the same number of white LEDs in both strings.
frequency (ie., 500Hz). Driving ELCD, ELED1, and ELED2
low initiates shutdown mode which disables all IC func-
tions and reduces quiescent current from the battery to
less than 2μA.
LED1 (Pin 19): Output for First LED String.
VLED (Pin 20): Output of the LED Switcher. Bypass VLED
with a low ESR, ESL ceramic capacitor (X5R type) of at
least 1μF. Keep PCB trace lengths as short and wide as
possible to minimize EMI and voltage overshoot.
SW2 (Pin 21): White LED Boost Switch. Connect a 3.3-
15μH inductor between SW2 and V_IN. This is the collector
of the internal NPN power switch. Connect an external
Schottky diode between SW2 and VLED. Keep PCB trace
lengths as short and wide as possible to minimize EMI
and voltage overshoot.
ELED2 (Pin 22): Enable and PWM Dimming Control Input
for the LED2 String. The LED2 string is disabled when this
pin is grounded. Digital dimming can be implemented
by driving the ELED2 pin between 0V and >1.2V at low
PROG (Pin 23): A single resistor (RPROG) between
PROG and GND sets the current in the LED strings. LED
current in mA is programmed by:
IL E D1 =
ILED2
=
⎛
⎝⎜
2 × 10⁶
R_PROG
⎞
⎠⎟
mA
A 100K resistor programs 20mA in each string. Analog
dimming can be implemented by connecting a second
resistor between PROG and a control voltage.
ELED1 (Pin 24): Enable and Pulse Dimming Control Input
for the LED1 String. For applications with five or fewer
LEDs, better efficiency is achieved by operating a single
LED string. For example, ELED1 = 1, ELED2 = 0, LED2 left
open circuit and the LED string connected to LED1.
3524f
7